Many devices for providing rotational force such as internal combustion engines and electrical motors operate most efficiently over a range of rotational speeds (usually measured in revolutions per minute or "r.p.m.") that is relatively narrow. Many applications for such devices however require both rotational speeds outside of the range and incremental variations of speed.
Previously rotational speeds in applications for such devices have been controlled by controlling the speed of the device, interspersing a transmission having a few different fixed ratios between the device and a driven component or combinations of both.
A common example is an automobile where speed is controlled both by varying the speed of the engine and using a transmission or gearbox in which various "drive ratios" can be selected. The "drive ratio" is determined by the number of revolutions of an input shaft into the transmission required to cause one revolution of the output shaft.
The two most common automotive transmissions are manually selected transmissions and automatically selected transmissions. More modern manually selected transmissions of the type referred to as gears connectable to an input shaft which transmits rotational force to a corresponding number of gears connectable to an output shaft through a "cluster gear" comprising a corresponding number of gearsets cut into a single member. Selection of ratios in a manual transmission is achieved by locking selected gears to the input and output shafts, the remaining gears being free to rotate without transmitting rotational force.
Most modern automatic transmissions use a series of planetary gearsets each capable of two ratios depending on how the components of the gearsets are constrained to move by hydraulically activated friction clutches referred to as "bands".
Both manual and automatic transmissions are quite complex and expensive because of the requisite number of accurately machined and fitted components.
Despite advances made in automotive transmissions, they are generally limited to anywhere from three to five ratios by space and cost considerations usually requiring a combination of engine speed and gear selection to adequately control the speed and power requirements of the automobile. This results both in automobile engines often being operated out of their optimal r.p.m. range and an undesirable jerk resulting from the interruption and resumption of rotational coupling between the input and output shafts required to change from one combination of gearsets to another.
Various attempts have been made in the past to provide transmissions wherein the drive ratio is continuously and non-incrementally varied without requiring coupling and decoupling of various gearsets. These attempts have generally been based on a design first built by Messrs. Daimler and Benz in 1886.
The Daimler-Benz continuously variable transmission ("C.V.T.") which is illustrated as "FIG. 1" basically used a rubber V-belt 1 riding between two opposed pairs of shallow angle cones 2. Moving each pair of cones toward each other (as indicated by arrows 4) would cause the belt to ride "higher" on the cones and in effect run on a pulley of larger diameter. Moving the cones apart (as indicated by arrows 3) would cause the belt to run "lower" on the cones and in effect run on a pulley of smaller diameter. Simultaneously moving one pair of cones toward each other while moving the other pair of cones away from each other would vary the relative drive ratios between the pairs of cones.
A problem with the Daimler-Benz design is that attempts to transmit significant amounts of torque result in slippage of the belts.
The reason that many C.V.T. designs rely on the Daimler-Benz principle is that the V-belt frictionally engages the cones thereby avoiding any problems associated with requiring toothed components to continually mesh with each other or with a chain despite diametrical changes which would ordinarily cause variations in the pitch of the teeth. However the transmission of significant amounts of torque is better achieved by components which mesh rather than by frictionally coupled components.